Electromagnetic Acoustic Transducers (EMATs) are commonly used for non-destructive testing of metal structures. The EMATs can measure wall or plate thickness, and can detect cracks or other non-uniformities within the specimen. Material flaws can be caused by fatigue, corrosion, erosion, pitting, or wear. Flaws can also result from a fault in the manufacturing process. An advantage of using EMAT compared to conventional ultrasonic transducers (UT) is that the ultrasonic energy is transferred directly into the material of the specimen. Furthermore, while conventional UT requires a coupling gel or liquid to transfer the energy from the transducer to the object to be inspected, the EMAT based technologies do not require the coupling gel or liquid.
FIG. 1 shows a conventional EMAT transducer. A magnet 3, which can be a permanent magnet or an electromagnet, generates a biasing magnetic field over a sensor coil 4 that carries an alternating electrical current. The sensor coil 4 is placed near the object 1 to be inspected (also referred to as a specimen or a pipe). Interaction between the biasing magnetic field and the electrical current in the sensor coil 4 ultimately produces a radiating acoustic field 2 within the object to be inspected. By the principle of reciprocity, the reflected acoustic field in the object 1 can interact with a biasing magnetic field to produce a magnetic field that can now induce another current in the sensor coil 4. This combined transmit and receive capability of the sensor coil 4 provides a method of acoustically detecting and measuring flaws in the object to be inspected.
EMAT transducers commonly use strong permanent magnets (e.g., the magnet 3) to produce the required biasing magnetic field. In some situations, the electromagnets replace the permanent (also referred to as “hard”) magnets. However the electromagnets require magnetizing coils with a large number of turns and need to maintain high currents for the duration of measurement. Because the magnetizing coil is an inductor, it also takes a certain amount of time for the current to reach the necessary levels for the magnetization. Additionally, the high electrical current generates heat that is difficult to dissipate. Therefore, the electromagnets typically need some form of cooling or are only usable for low duty-cycle applications where the ratio of the on-time to the off-time is small. Even if the duty cycle is low, heat dissipation can still be a problem if a single on-time event lasts long time. For wall thickness measurements, the on-time for single measurements might be on the order of 50 μs, however for a long range guided wave application, the sensor may need to be active for 10's milliseconds. In addition, to minimize resistance losses over long distances, wiring needs to have large diameter and thick electrical insulation, resulting in bulky cables that are difficult to handle. As a result, the use of hard magnets is preferred for most applications.
Conventional EMATs that use hard magnets also have some drawbacks. For example, EMAT based tools need to be shipped around the world in a timely fashion. However, transportation of the magnetic material by aircraft is regulated because the magnetic materials are considered hazardous. For example, according to the Federal Aviation Administration (FAA) regulations, the shipper of the magnetized materials must ensure that the package generates a magnetic field strength of less than 0.00525 gauss when measured at 15 feet from any surface of the package. For the packages having magnetic field less than 0.00525 gauss when measured at 15 feet, but greater than 0.002 gauss when measured 7 feet from the package, the package must be labeled “magnetic.” As a comparison, the Earth's magnet field strength is approximately 0.5 Gauss. Therefore, in many cases the shipping box must be shielded, or the strength of the magnetic field must be reduced by putting magnetic shorting bars or “keepers” between the poles of the magnet. This is expensive, and results in a heavier shipment and a more time-consuming process for the operator or the shipper.
Furthermore, when the EMAT equipment is transported either by hand or machine to the object to be inspected, care must be taken to assure that ferromagnetic materials near the equipment are kept at a safe distance. Additionally, if the EMAT transducer is handheld or integrated into a portable tool that is attached to a ferromagnetic plate or inserted into a ferromagnetic pipe, the tool is difficult to handle due to the attraction between the strong magnet(s) integrated into the tool and the plate/pipe. Strong magnetic force may create safety issues as well, especially if fingers or other body parts get trapped between the magnets or between the magnets and ferromagnetic objects.
During the transport or use of the EMAT tools, the magnetic fields attract ferromagnetic debris that needs to be removed periodically to maintain proper operation of the tool. This cleaning is a time-consuming process, and may be difficult to perform. In addition, the overall tool design may need special provisions for the cleaning, for example by assuring that there are no slots or empty space where ferromagnetic debris accumulates.
FIG. 2A is schematic view of a prior art electro-permanent magnet (EPM) 40A in the operational configuration. EPMs are known alternatives to electromagnets and hard magnets. The transducer 40A includes a hard magnet 7 and an electromagnet 9 that can be polarized using a magnetizing coil 10. FIG. 2A illustrates the electromagnet 9 that is polarized to have N-S orientation that corresponds to that of the hard magnet 7. As a result, a magnetic flux 13 is strengthened. Yoke 5 conducts the magnetic flux 13 into the test specimen 1.
FIG. 2B is schematic view of a prior art electro-permanent magnet (EPM) 40B in the transportation configuration. The polarity of the electromagnet 9 is now inverted to be opposite to that of the hard magnet 7, i.e., the electromagnet 9 is switchable. As a result, the magnetic flux 13 travels from one magnet to another through the yoke 5 because this is a path of the least resistance for the magnetic flux. Since the magnetic flux 13 is generally constrained within the yoke 5, the leakage of the magnetic flux 13 outside of the EPM is minimized, and the electro-permanent magnet (EPM) 40B is suitable for transportation.
However, the electro-permanent magnet (EPM) 40A/40B is relatively bulky, resulting in increased cost/difficulty of transportation. Accordingly, there remains a need for compact EMAT tools that can produce strong magnetic field when the EMAT operates, while reducing or eliminating the leakage of the magnetic field when the EMAT is not in operation.